i have a 1983 900 seca that is driving me nuts. sometimes it will start and run fine for awhile. if it dies it won't start unless i crank on it for awhile. other times it won't start at all. what really puzzles me is that it will only hit after i let off of the start button. the fire does seem kinda weak. i've pulled the number one plug out to check the fire and it has fired up while testing it then wouldn't hit when i put it back in. i don't think it's a fuel problem. any ideas would be helpful

What kind of condition is your battery in? The starters in the XJs are really high torque and really efficient.. Too efficient.. The starter will be able to crank the motor, even if there isn't enough power in the battery to fire the ignition. Just for the hell of it, try giving it a jumpstart from your car/truck. Betcha it'll work.. ;) You'll also want to test your charging system to make sure it's putting out what it's supposed to. If the regulator or an alternator winding is going bad, it may not be putting out enough power to run the bike and keep the battery fully charged.